Two-variable strip chart recorder

ABSTRACT

A single-pen, two-variable recorder using a heat-sensitive paper and a heated pen. The paper moves at a fixed rate and ordinarily a first continuous signal is recorded. At periodic intervals, samples are taken of a second signal and the pen is used to scribe bars (in the form of a bar graph) with the length of each bar corresponding to the amplitude of a respective sample. While a bar is being recorded, the pen moves relatively fast. This results in lighter lines for the bar graph than for the continuous trace, and allows a fast sampling rate relative to the paper speed without having the bar graph obscure the graticule markings on the paper.

United States Patent Harris et al.

[54] TWO-VARIABLE STRIP CHART RECORDER [72] Inventors: George J. Harris,Framingham; William D. Richards, Medway, both of Mass.

[73] Assignee: American Optical Corporation,

Southbridge, Mass.

[22] Filed: Jan. 26, 1971 [21] Appl. No.: 109,904

[52] US. Cl ..346/62, l28/2.06 G, 346/76 [51] Int. Cl. ..G0ld 9/34 [58]Field of Search ..346/62, 34, 76, 33 ME;

[56] References Cited UNITED STATES PATENTS 3,478,364 11/1969 Frank..346/62 51 Aug. 1,1972

3,573,843 4/1971 Solley ..346/62 Primary Examiner-Joseph W. HartaryAttorney-Robert .1. Bird, Joel Wall, William C.

Nealon and Noble S. Williams [57] ABSTRACT A single-pen, two-variablerecorder using a heat-sensitive paper and a heated pen. The paper movesat a fixed rate and ordinarily a first continuous signal is recorded. Atperiodic intervals, samples are taken of a second signal and the pen isused to scribe bars (in the form of a bar graph) with the length of eachbar corresponding to the amplitude of a respective sample.

While a bar is being recorded, the pen moves relatively fast. Thisresults in lighter lines for the bar graph than for the continuoustrace, and allows a fast sampling rate relative to the paper speedwithout having the bar graph obscure the graticule markings on thepaper.

28 Claims, 5 Drawing Figures IATENTEIJMIB nan 358L774 susmnrs T0 PENLIFT PEN LIFT DRIVER III-j FIG. 5

INVENTORS GEORGE J. HARRIS WILLIAM D. RICHARDS MWZZ AGENT TWO-VARIABLESTRIP CHART RECORDER This invention relates to strip chart recorders,and

more particularly to single-pen, two-variable strip chart recorders.

In a strip chart recorder, a paper strip is continuously fed at aconstant rate from a roll above a recording surface. A pen bears againstthe paper on the recording surface and is driven, in accordance with thesignal to be recorded, in a direction perpendicular to the direction ofpaper movement. Thus, on the final recording, the time axis is along thelength of the paper strip and the variable is represented by thedistance of each point on the trace from a base line in a directionperpendicular to the time axis. The distance of each point from the baseline is dependent upon the mag nitude of the signal (i,e., the magnitudeof the pen deflection) at one instant during the recording.

If two signals are to be recorded simultaneously, two pens may beprovided with each signal controlling the deflection of a respectivepen. In such a case, the paper must be sufficiently wide so that thesignals do not cause the traces to overlap; otherwise, the two pens willhit each other and neither signal will be recorded properly. Onedisadvantage of two-pen recorders is that they are generally moreexpensive than single-pen recorders. Another disadvantage is that thetwo traces are often separated so far from each other that it isdifficult to correlate points on the two traces.

Instead of using two pens, it is possible to use a single pen which istime-shared by two (or more) channels. The two signals to be recordedcan be .applied alternately to the pen deflection drive circuit. The pennormally is kept at a small distance from the paper. Each time one ofthe two signals is applied to the pen deflection drive circuit and thepen moves to a position to be marked on the respective trace, the penmay be brought into contact with the paper to produce a mark. If themovement of the pen up and down relative to the paper is synchronized tothe sampling of the two signals, each trace will consist of a series ofmarks corresponding to the respective signal. Instead of moving the penup and down relative to the paper, it is also possible to employ aspecially treated paper which is sensitive to high voltage discharges.The pen may be kept at a fixed distance from the paper, and to record amark a high voltage pulse is applied to the pen. (This type of recordingis known as electrostatic recording".) The major problem with this formof recording is that a sampled trace is almost always less clear than acontinuous trace.

Instead of recording two sampled traces, it is often advantageous with asingle-pen recorder to develop two different kinds of traces. One ofthese may be the usual type of continuous recording. The other may be abar or a line graph which consists of a series of bars perpendicular tothe direction of movement of the paper. Each bar has one of its ends ona base line, and the height of the bar represents the amplitude of therespective signal at the time the bar was drawn. The bars should beseparated from each other by some finite distance so that the height ofeach bar at any time can be determined. While the first signal isrecorded almost continuously on the paper strip (the discontinuities asa result of the drawing of the bars at periodic intervals may not evenbe observable), the second signal is sampled only periodically with eachsample causing a bar of proportional length to be recorded. One of theadvantages of this type of recording is that the two traces may overlap,that is, the continuous trace may be drawn right through the series ofbars. In this manner, correlation between the two signals is easilyobserved. (For example, if on one section of the paper all of the barsare relatively long while the amplitude of the continuous trace is quitelow, this relationship is immediately evident.) Furthermore, because thetwo signals result in completely different types of recording, therecordings can be distinguished quite easily from each other.

This kind of dual recording with a single pen has not been exploitedcommercially on a large scale for the following reason. The individualbars or lines must be separated from each other in order for the bargraph to be interpreted. But unless the paper is moved very fast(thereby consuming large quantities), or unless the signal to be sampledis sampled at widely separated intervals, the bars will be very close toeach other. The paper strip is almost always provided with horizontaland vertical graticule lines so that the values of the abscissa (time),and ordinate (signal amplitude) of any point on the continuous trace orthe top of any bar, can be determined easily. But if the bars arerecorded close to each other, they may completely obscure the verticaland horizontal graticule lines. In such a case, it is very difficult todetermine the length of any bar. And if the bars are to be separatedsufficiently so that the graticule lines are not masked, either thepaper must be fed at a fast rate,or the sampled signal must be sampledonly infrequently.

It is an object of our invention to provide a singlepen, two-variablestrip chart recorder for recording a continuous trace and a bar graphwhich does not suffer from the disadvantages described above.

Briefly, in the illustrative embodiment of our invention, we employ aconventional strip chart recorder which utilizes heat-sensitive paper.The pen is a heated stylus which burns the paper when it comes intocontact with it. The stylus is normally employed to record a continuoustrace of a first signal. Periodically, the stylus is lifted from thepaper and a sample is taken of a second signal. The stylus is deflectedto the top of the next line to be drawn and then quickly moves acrossthe width of the paper to the base line. When the base line is reached,the stylus is lifted and returned to a position determined by the firstsignal, at which time it is lowered once again and is used to record thefirst signal.

The density (darkness) of any line recorded on the heat-sensitive paperdepends upon the speed of stylus movement across the paper. The fasterthe speed of the stylus, the lighter the line which is drawn. Althoughthe pen moves slowly relative to the paper while the continuous trace isbeing drawn, it moves very fast relative to the paper when a bar isbeing drawn following the sampling of the second signal. Because the penmoves so fast, the intensity of the parallel lines in the bar graph ismuch lower than the intensity of the continuous trace. (The intensity isproportional to the width of each line, the width increasing as thespeed of the stylus decreases since every point on the paper passed bythe I stylus is heated for a longer period of time.) The low-intensitylines in the bar graph do not obscure the graticule lines even thoughthe lines are relatively close to each other in the case of a highsampling rate.

It is a feature of our invention, in the illustrative embodimentthereof, to utilize the single pen in a heat-sensitive type strip chartrecorder most of the time for recording a first signal and forperiodically recording line lengths representative of samples of asecond signal, and moving the pen relative to the paper much fasterduring the line recording than during the continuous recording.

Further objects, features and advantages of our in vention will becomeapparent upon consideration of the following detailed description inconjunction with the drawing, in which:

FIG. 1 depicts the mechanical elements in the illustrative embodiment ofthe invention;

FIG. 2 is a side view of the apparatus of FIG. 1;

FIG. 3 depicts schematically the electronic circuit in the illustrativeembodiment of the invention;

FIG. 4 is a timing diagram; and

FIG. 5 illustrates a typical dual-signal trace of the type which can bemade with the strip chart recorder of v the invention.

Referring to FIGS. 1 and 2, pen l6 isattached to shaft 14 of pen motor12. Conductor 32 is connected to the' output of a pen driver (pen driver78 on FIG. 3) and depending on the magnitude of the pen driver signalshaft 14 rotates as shown by arrow 15. The larger the signal, thegreater the clockwise rotation of the shaft as viewed from above themotor. The tip of pen 16 is maintained heated as is known in the art asit moves back and forth across paper 10. The paper is fed at a constantrate from a roll (not shown) over rod 17 in the direction shown by arrow19. As the paper moves, the heated tip of the pen burns away the plasticcoating on the paper to form a trace.

Pen lift solenoid 24 has an input terminal connected to the output of apen lift driver (pen lift driver 38 on FIG. 3). Clapper 24a is hinged tothe solenoid as shown and is normally in the position shown in FIGS. 1and 2. To the front of the clapper there is connected a rod 28. The tipof this rod is contained in a hook c at one end of wire bail 20.

Shaft 18 is fixed as shown, and wire bail 20 is bent so that it ishinged to the shaft as shown at 20a and 20d. A portion 20b of the wirebail is disposed underneath pen 16. When the pen lift solenoid isoperated, clapper 24a moves in the direction shown by arrow 26 to causerod 22 to move in the direction shown by arrow 28. This causes the wirebail to rotate (counterclockwise in FIG. 2) around shaft 18 since thewire bail is hinged at 20a and 20d. When the wire bail rotates in thismanner, section 20b bears up against the pen and lifts it from thepaper. At this time, recording ceases.

The pen deflection drive is normally controlled by a slowly-changingsignal (A) and the tip of the pen rests on the paper. Slow motion of theheated pen with respect to the paper causes a dark line to be traced onthe heat-sensitive paper. This dark line is the heavy trace in FIG. 5.(The traces of FIG. 5 are recorded as the strip of paper moves to theleft.) At l-minute intervals the pen is lifted from the paper, anda'sample of a second signal (B) is coupled to the pen deflection drive.The magnitude of the sample causes the pen to move to the top of the barto be drawn. The pen is then lowered to the paper and the sample ofsignal B is allowed to decay to zero in a relatively short time (e.g.,less than one second). As will be described, this is accomplished bystoring the sample on a capacitor and discharging the capacitor with aconstant current source. As the voltage on the capacitor decays, the pentraces out a vertical line, starting at a point proportional to the fullmagnitude of the sample and ending at the base line of the bar graph. Atthe conclusion of the stroke, the pen is lifted once again and the pendeflection drive is connected to signal A once again. The pen is loweredand signal A is recorded continuously for another one-minute interval.

The overall plot generated, as shown inFlG. 5, consists of a series ofclosely spaced vertical bars (samples of signal B) which are intersectedby a darker continuous line (signal A). The heights of the individualbars correspond to the amplitudes of signal B as sampled at one-minuteintervals. The position of each point on the continuous darker lineabove the base line reflects the magnitude of signal A at the timerepresented on the time (horizontal) axis. The relative positions of thebars and the continuous line are determined by bias controls to bedescribed in connectionwith FIG. 3. It is possible to have thecontinuous trace above or below the bars, or superimposed on them (asshown). The bars are much lighter (less dense) than the continuous tracebecause of the rapid pen movement when a line is formed; theheat-sensitive plastic does not have enough time to fully melt off fromthe paper. Thus the original graticule markings on the paper remainvisible in the background with the bar graph superimposed on them, eventhough samples of signal B are taken frequently relative to the rate ofpaper feed.

The circuit for controlling the operation of pen lift solenoid 24 andpen motor 12 is shown in FIG. 3. The system timing is controlled byl-minute timer 34 and sequential pulse generator 36. At l-minuteintervals, the timer triggers the operation of the pulse generator. Thepulse generator then generates six pulses on output conductors Tl-T6.The pulse waveforms are shown in FIG. 4. Each of the T1-T3, T5 and T6pulses is approximately 50 milliseconds in duration. Pulse T4 isapproximately 800 milliseconds in duration. The six pulses are generatedin sequence. (As is known by those skilled in the art, six successiveone-shot multivibrators can be used for this purpose.) Following thegeneration of the T6 pulse, all six of the output conductors remain lowin potential for approximately 59 seconds until the next 1- minute timerpulse starts the cycle once again. Thus all of the operations which arecontrolled by pulses Tl-T6 occur in about 1 second.

The pen-lift signal, as shown in FIG. '4, is a function of the pulses onconductors T1, T2, T5 and T6. With the generation of pulse T1 at thestart of each l-minute interval, the pen rises. (A high level for thepen-lift signal is an indication that the pen is raised. Prior to thistime the pen has been tracing signal A. But immediately after it israised from the paper, during the time that the T2 pulse is generatedthe pen travels to the top of the next bar to be drawn. The pen is movedto a position from the base line which is proportional to the amplitudeof the sample to be represented by the line now to be drawn. At thestart of the T3 pulse, the pen is allowed to fall. Thereafter, duringthe 800-millisecond T4 pulse, the pen moves quickly toward the base lineand scribes a bar whose length represents a sample of signal B. At thestart of the T5 pulse, the pen rises. Once the pen is raised, it travelsto a position dependent upon the amplitude of signal A during the5-millisecond T6 pulse. Thereafter, the pen drops once again and beginsto scribe signal A. After approximately 59 seconds of recording signalA, the cycle starts over once again.

In the illustrative embodiment of the invention, the continuous trace isa plot of heart rate. Each time a QRS complex is detected (this occursabout once or twice per second when a patient is being monitored), apulse is generated. Each pulse is of the same'amplitude and width, and.the succession of pulses are integrated. Thus the output of theintegrator (filter) is a signal (A) proportional to the heart rate. Thesystem also monitors ventricular premature beats (VPBs) and once aminute the number of VPBs detected during the preceding one-minuteinterval is converted to an analog signal. This analog signal (B) isused to control the length of a bar recorded in the bar graph.

Referring to FIG. 4, when the T2 pulse is generated channel B issampled. This means that the number of VPBs detected during thepreceding l-minute interval is converted to a voltage level. It is thislevel that causes the pen to travel to the top of the bar next to bedrawn. The sample channel B signal terminates with the leading edge ofthe T6 pulse. (This is merely a matter of convenience; the signal is notrequired after the pen has traveled to the top of the bar to be drawn.)The analog signal is developed in the first place by counting the VPBsduring each one-minute interval and then converting them to an analoglevel which is used to move the pen to the top of the bar to be drawn.The counter must be reset after each sample is taken so that a new countcan begin. The reset VPB counter signal (the T3 pulse) shown in FIG. 4resets the counter at the same time that the pen is allowed to falltoward the paper after it has been moved to the top of the bar to bedrawn.

The discharge circuit signal on FIG. 4, as will be described withreference to FIG. 3, is simply a circuit which allows the signal sampleon the storage capacitor to decay. As the voltage on the capacitordecays, a bar is drawn. The discharge circuit signal is thereforegenerated during that time that the bar is actually drawn, that is,during the T4 pulse.

Referring to the circuit of FIG. 3, VPB pulse source 44 generates apulse each time a VPB is detected, and the pulse increments the count inVPB counter 46. The count is reset when a T3 pulse from sequential pulsegenerator 36 is applied to the counter reset input. The output of thecounter is extended to the input of digitaltoanalog converter 48 and theoutput of the converter is an analog signal which is fed throughnormally closed switch 50 to capacitor 52 for controlling the recordingof a proportional line length when the capacitor voltage is used todrive the pen.

The R-wave one-shot multivibrator 76 generates a pulse each time theR-wave of an ECG waveform is detected. The pulses are smoothed by filter74 and consequently the input to operational amplifier 68 thoughresistor 86 is a signal whose magnitude is proportional to the averageheart rate. Potential source 72 is connected through potentiometer 70and resistor 82 to ground, with the junction of the potentiometer andthe resistor also being connected to the input of operational amplifier68. The setting of the potentiometer controls the bias of signal A, thatis, the average value of the dark continuous trace relative to the baseline.

The output of the operational amplifier is extended to switch 66. Thisswitch is normally closed and allows signal A (the output of operationalamplifier 68) to be extended to the input of pen driver 78. The outputof the pen driver is extended over conductor 32 to the pen motor(FIG. 1) and thus in the absence of a sample channel B" signal onconductor 42 a continuous recording is made of the average heart rate.

Each time one-minute timer 34 applies a pulse at the input-of sequentialpulse generator 36, the sequential pulses shown on FIG. 4 are generatedon conductors Tl-T6. Conductors T1, T2, T5 and T6 are extended torespective inputs of OR gate 38. This OR gate functions as the pen liftdriver and its output conductor is extended to the pen lift solenoid(FIG. 1). The solenoid is operated, thereby lifting the pen, during theT1, T2, T5

and T6 pulse intervals. This is shown by the pen-lift signal of FIG. 4.

The T2 conductor is also coupled to the set input of flip-flop 40. Theoutput of the flip-flop is ordinarily low but goes high when it is set.Thus the sample channel B signal goes high at the start of the T2 pulse.When the signal goes high, switch 66 is opened and consequently signal Ais no longer extended to the input of the pen driver. The high signal onconductor 42 causes switch 50 to open. This normally closed switchcharges the capacitor 52 to the voltage representing the top of the barto be drawn. When the switch opens, the capacitor holds the charge.Since the capacitor is connected to the input of operational amplifier62 through resistor 85, the output of the operational amplifier isproportional to the magnitude of the bar. Potential source 58,potentiometer 60 and resistor 84 function as r a bias control for thebar recording, and determine the level of the base line of the bargraph.

The high signal on conductor 42 is also extended to the input of switch64. This switch is closed only when switch 66 is open, and extends thesignal at the output of operational amplifier 62 to the input of pendriver 78. Consequently, as soon as flip-flop 40 is set (at the leadingedge of the T2 pulse), the pen, which has already been lifted from thepaper, moves to a position which will be at the top of the bar now to bedrawn.

During pulse T3, OR gate 38 does not operate and the pen falls. The T4pulse does not operate OR gate 38 either, but it does operate dischargecircuit 56. The discharge circuit is simply a switch which allowscurrent to flow from constant current source 54 to ground. As thecurrent flows, capacitor 52 is discharged. The discharge, even for amaximum voltage sample on capacitor 52, requires less than 800milliseconds. As the capacitor discharges, its voltage decreases and theinput to the pen driver falls. The pen thus scribes a bar on the paper.The capacitor is discharged very fast and it is the rapid movement ofthe pen as the bar is drawn which causes a relatively low-intensity lineto be drawn.

At the end of the T4 pulse, when the T5 pulse is generated, OR gate 38operates once again. The pen is lifted from the paper. The leading edgeof the T6 pulse resets flip-flop 40 so that conductor 42 goes low inpotential. At this time switch 64 opens, and switches 50 and 66 closeso-that signal A is applied once again to the input of the pen driver.During the T6 pulse, the pen is still in a raised position since the penshould move before it is lowered from the bottom of the barnext bar isto be drawn.

It should be noted that whenever the pen is traveling toward a newposition during pulse time T2 as it travels to the top of the bar to bedrawn and during pulse time T6 as it travels to the signal A positionthe pen is raised. This is to avoid interference between the tworecordings. Each pen travel is preceded by a pen rise interval (pulsesT1 and T5), and each pen travel is succeeded by a pen falling interval(during pulse T3, and the initial interval immediately after pulse T6),which in turn is followed by the scribing of a signal (during pulse T4,and the 59 seconds or so prior to the next one-minute pulse).

Although the invention has been described with reference to a particularembodiment, it is to be understood that this embodiment is merelyillustrative of the application of the principles of the invention. Forexample, instead of using heat-sensitive paper and a hot pen, it ispossible to use inkpen recorders. If a light ink is used, the fast speedof the pen when a bar is being drawn will'result in a light line beingdrawn (as opposed to the much heavier line which is drawn for signal A).In this case also the graticule markings show through the bar graph evenfor a paper feed rate which is low relative to the sampling rate. Also,instead of discharging capacitor 52 to control the drawing of a line, itis possible to maintain the sample voltage across the capacitor as theline is drawn. In such case, a second capacitor can be provided, andduring pulse T4 this capacitor can be charged (rather than discharged).This second capacitor is actually used to drive the pen throughamplifier 62. The line is thus drawn upward from the base line. As soonas the voltage across the second capacitor equals the voltage acrosscapacitor 52, a comparator can be used to cause the charging toterminate (thus determining the upper end of the line). To reset thecircuit, the second capacitor must be discharged after each cycle andthis can be accomplished during pulse T6. Thus it is to be understoodthat numerous modifications may be made in the illustrative embodimentof the invention and other arrangements may be devised without departingfrom the spirit and scope of the invention.

What we claim is:

1. A strip chart recorder comprising means for moving continuously apaper strip, penmeans normally for recording a trace of a first signalon the moving paper strip, means for periodically sampling a secondsignal, means operative after the sampling of said second signal forcontrolling said pen means to record a line of a bar graph the length ofwhich is dependent upon the magnitude of a sample taken previously, andmeans for causing said pen means to move substantially faster whenrecording a line of said bar graph than when recording said trace suchthat the final recording on said paper strip has an intensity for thetrace of said first signal which is substantially greater than theintensity of the lines in said bar graph.

2. A strip chart recorder in accordance with claim 1 wherein said paperis heat-sensitive paper and said pen means includes a heated tip, andfurther including means for lifting said pen means from said paper bothprior to and after the drawing on said paper of any line of said bargraph.

3. A strip chart recorder in accordance with claim 2 wherein saidcontrolling means causes said pen to be moved to a position on saidpaper corresponding to one end of any line to be drawn, and thereaftercauses said pen to move from said position on said paper to the otherend of said line to be drawn on said paper a distance corresponding tothe amplidude of the respective sample.

4. A strip chart recorder in accordance with claim 3 wherein saidcontrolling means includes a capacitor, means for applying a voltageacross said capacitor which is dependant upon the amplitude of a sample,and means for controlling the drawing of a line by said pen means of alength which is proportional to the magnitude of the voltage across saidcapacitor.

5. A strip chart recorder in accordance with claim 4 further includingmeans for adjusting the base line of said bar graph and the averagevalue of the trace recorded on said paper relative to each other suchthat the separation and superposition of said trace and said bar graphcan be varied.

6. A strip chart recorder in accordance with claim 5 wherein said paperincludes mutually perpendicular graticule markings, said pen means ismoved fast enough during the recording of a line such that saidgraticule markings are readily observable in the region of said bargraph, and the intensity of said trace is great enough such that thedrawing of said lines with substantially the same intensity wouldobscure said graticule markings.

7. A strip chart recorder in accordance with claim 6 wherein said firstsignal is derived from the continuous cardiographic monitoring of apatient, and the samples of said second signal are proportional to acount of the number of events of a specified nature which occur betweenthe taking of successive samples.

8. A strip chart recorder in accordance with claim 3 wherein said firstsignal is derived from the continuous cardiographic monitoring of apatient, and the samples of said second signal are proportional to acount of the number of events of a specified nature which occur betweenthe taking of successive samples.

9. A strip chart recorder in accordance with claim 3 wherein said paperincludes mutually perpendicular graticule markings, said pen means ismoved fast enough during the recording of a line such that saidgraticule markings are readily observable in the region of said bargraph, and the intensity of said trace is great enough such that thedrawing of said lines with substan tially the same .intensity wouldobscure said graticule markings.

10. A strip chart recorder in accordance with claim 3 further includingmeans for adjusting the base line of said bar graph and the averagevalue of the trace recorded on said paper relative to each other suchthat the separation and superposition of said trace and said bar graphcan be varied.

11. A strip chart recorder in accordance with claim 10 wherein saidpaper includes mutually perpendicular graticule markings, said pen meansis moved fast enough during the recording of a line such that saidgraticule markings are readily observable in the region of said bargraph, and the intensity of said trace is great enough such that thedrawing of said lines with substantially the same intensity wouldobscure said graticule markings.

12. A strip chart recorder in accordance with claim 11 wherein saidfirst signal is derived from the continuous cardiographic monitoring ofa patient, and the samples of said second signal are proportional to acount of the number of events of a specified nature which occur betweenthe taking of successive samples.

13. A strip chart recorder in accordance with claim 10 wherein saidfirst signal is derived from the continuous cardiographic monitoring ofa patient, and the samples of said second signal are proportional to acount of the number of events of a specified nature which occur betweenthe taking of successive samples.

14. A strip chart recorder in accordance with claim 2 further includingmeans for adjusting the base line of said bar graph and the averagevalue of the trace recorded on said paper relative to each other suchthat the separation and superposition of said trace and said bar graphcan be varied.

15. A strip chart recorder in accordance with claim 14 wherein saidpaper includes mutually perpendicular graticule markings, said pen meansis moved fast enough during the recording of a line such that saidgraticule markings are readily observable in the region of said bargraph, and the intensity of said trace is great enough such that thedrawing of said lines with substantially the same intensity wouldobscure said graticule markings.

16. A strip chart recorder in accordance with claim 15 wherein saidfirst signal is derived from the continuous cardiographic monitoring ofa patient, and the samples of said second signal are proportional to acount of the number of events of a specified nature which occur betweenthe taking of successive samples.

17. A strip chart recorder in accordance with claim 14 wherein saidfirst signal is derived from the continuous cardiographic monitoring ofa patient, and the samples of said second signal are proportional to acount of the number of events of a specified nature which occur betweenthe taking of successive samples.

18. A strip chart recorder in accordance with claim 1 wherein saidcontrolling means causes said pen to be moved to a position on saidpaper corresponding to one end of any line to be drawn, and thereaftercauses said pen to move from said position on said paper to the otherend of said line to be drawn on said paper a distance corresponding tothe amplitude of the corresponding sample.

19. A strip chart recorder in accordance with claim 18 wherein saidcontrolling means includes a capacitor, means for applying a voltageacross said capacitor which is dependent upon the amplitude of a sample,and means for controlling the drawing of a line by said pen means of alength which is proportional to the magnitude of the voltage across saidcapacitor.

20. A strip chart recorder in accordance with claim 19 further includingmeans for adjusting the base line of said bar graph and the averagevalue of the trace recorded on said paper relative to each other suchthat the separation and superposition of said trace and said bar graphcan be varied.

21. A strip chart recorder in accordance with claim 20 wherein saidpaper includes mutually perpendicular graticule markings, said pen meansis moved fast enough during the recording of a line such that saidgraticule markings are readily observable in the region of said bargraph, and the intensity of said trace is great enough such that thedrawing of said lines with substantially the same intensity wouldobscure said graticule markings. V

22. A strip chart recorder in accordance with claim 21 wherein saidfirst signal is derived from the continuous cardiographic monitoring ofa patient, and the samples of said second signal are proportional to acount of the number of events of a specified nature which occur betweenthe taking of successive samples.

23. A strip chart recorder in accordance with claim 1 further includingmeans for adjusting thebase line of said bar graph and the average valueof the trace recorded on said paper relative to each other such that theseparation and superposition of said trace and said bar graph can bevaried.

24. A strip chart recorder in accordance with claim 23 wherein saidpaper includes mutually perpendicular graticule markings, said pen meansis moved fast enough during the recording of a line such that saidgraticule markings are readily observable in the region of said bargraph, and the intensity of said trace is great enough such that thedrawing of said lines with substantially the same intensity wouldobscure said graticule markings.

25. A strip chart recorder in accordance with claim 24 wherein saidfirst signal is derived from the continuous cardiographic monitoring ofa patient, and the samples of said second signal are proportional to acount of the number of events of a specified nature which occur betweenthe taking of successive samples.

26. A strip chart recorder in accordance with claim 1 wherein said paperincludes mutually perpendicular graticule markings, said pen means ismoved fast enough during the recording of a line such that saidgraticule markings are readily observable in the region of said bargraph, and the intensity of said trace is great enough such that thedrawing of said lines with substantially the same intensity wouldobscure said graticule markings.

27. A strip chart recorder in accordance with claim 26 wherein saidfirst signal is derived from the continuous cardiographic monitoring ofa patient, and the samples of said second signal are proportional to acount of the number of events of a specified nature which occur betweenthe taking of successive samples.

28. A strip chart recorder in accordance with claim 1 wherein said firstsignal is derived from the continuous cardiographic monitoring of apatient, and the samples of said second signal are proportional to acount of the number of events of a specified nature which occur betweenthe taking of successive samples.

1. A strip chart recorder comprising means for moving continuously apaper strip, pen means normally for recording a trace of a first signalon the moving paper strip, means for periodically sampling a secondsignal, means operative after the sampling of said second signal forcontrolling said pen means to record a line of a bar graph the length ofwhich is dependent upon the magnitude of a sample taken previously, andmeans for causing said pen means to move substantially faster whenrecording a line of said bar graph than when recording said trace suchthat the final recording on said paper strip has an intensity for thetrace of said first signal which is substantially greater than theintensity of the lines in said bar graph.
 2. A strip chart recorder inaccordance with claim 1 wherein said paper is heat-sensitive paper andsaid pen means includes a heated tip, and further including means forlifting said pen means from said paper both prior to and after thedrawing on said paper of any line of said bar graph.
 3. A strip chartrecorder in accordance with claim 2 wherein said controlling meanscauses said pen to be moved to a position on said paper corresponding toone end of any line to be drawn, and thereafter causes said pen to movefrom said position on said paper to the other end of said line to bedrawn on said paper a distance corresponding to the amplidude of therespective sample.
 4. A strip chart recorder in accordance with claim 3wherein said controlling means includes a capacitor, means for applyinga voltage across said capacitor which is dependant upon the amplitude ofa sample, and means for controlling the drawing of a line by said penmeans of a length which is proportional to the magnitude of the voltageacross said capacitor.
 5. A strip chart recorder in accordance withclaim 4 further including means for adjusting the base line of said bargraph and the average value of the trace recorded on said paper relativeto each other such that the separation and superposition of said traceand said bar graph can be varied.
 6. A strip chart recorder inaccordance with claim 5 wherein said paper includes mutuallyperpendicular graticule markings, said pen means is moved fast enoughduring the recording of a line such that said graticule markings arereadily observable in the region of said bar graph, and the intensity ofsaid trace is great enough such that the drawing of said lines withsubstantially the same intensity would obscure said graticule markings.7. A strip chart recorder in accordance with claim 6 wherein said firstsiGnal is derived from the continuous cardiographic monitoring of apatient, and the samples of said second signal are proportional to acount of the number of events of a specified nature which occur betweenthe taking of successive samples.
 8. A strip chart recorder inaccordance with claim 3 wherein said first signal is derived from thecontinuous cardiographic monitoring of a patient, and the samples ofsaid second signal are proportional to a count of the number of eventsof a specified nature which occur between the taking of successivesamples.
 9. A strip chart recorder in accordance with claim 3 whereinsaid paper includes mutually perpendicular graticule markings, said penmeans is moved fast enough during the recording of a line such that saidgraticule markings are readily observable in the region of said bargraph, and the intensity of said trace is great enough such that thedrawing of said lines with substantially the same intensity wouldobscure said graticule markings.
 10. A strip chart recorder inaccordance with claim 3 further including means for adjusting the baseline of said bar graph and the average value of the trace recorded onsaid paper relative to each other such that the separation andsuperposition of said trace and said bar graph can be varied.
 11. Astrip chart recorder in accordance with claim 10 wherein said paperincludes mutually perpendicular graticule markings, said pen means ismoved fast enough during the recording of a line such that saidgraticule markings are readily observable in the region of said bargraph, and the intensity of said trace is great enough such that thedrawing of said lines with substantially the same intensity wouldobscure said graticule markings.
 12. A strip chart recorder inaccordance with claim 11 wherein said first signal is derived from thecontinuous cardiographic monitoring of a patient, and the samples ofsaid second signal are proportional to a count of the number of eventsof a specified nature which occur between the taking of successivesamples.
 13. A strip chart recorder in accordance with claim 10 whereinsaid first signal is derived from the continuous cardiographicmonitoring of a patient, and the samples of said second signal areproportional to a count of the number of events of a specified naturewhich occur between the taking of successive samples.
 14. A strip chartrecorder in accordance with claim 2 further including means foradjusting the base line of said bar graph and the average value of thetrace recorded on said paper relative to each other such that theseparation and superposition of said trace and said bar graph can bevaried.
 15. A strip chart recorder in accordance with claim 14 whereinsaid paper includes mutually perpendicular graticule markings, said penmeans is moved fast enough during the recording of a line such that saidgraticule markings are readily observable in the region of said bargraph, and the intensity of said trace is great enough such that thedrawing of said lines with substantially the same intensity wouldobscure said graticule markings.
 16. A strip chart recorder inaccordance with claim 15 wherein said first signal is derived from thecontinuous cardiographic monitoring of a patient, and the samples ofsaid second signal are proportional to a count of the number of eventsof a specified nature which occur between the taking of successivesamples.
 17. A strip chart recorder in accordance with claim 14 whereinsaid first signal is derived from the continuous cardiographicmonitoring of a patient, and the samples of said second signal areproportional to a count of the number of events of a specified naturewhich occur between the taking of successive samples.
 18. A strip chartrecorder in accordance with claim 1 wherein said controlling meanscauses said pen to be moved to a position on said paper corresponding toone end of any line to be drawn, and thereafter causes said pen to movefrom said position on said paper to the Other end of said line to bedrawn on said paper a distance corresponding to the amplitude of thecorresponding sample.
 19. A strip chart recorder in accordance withclaim 18 wherein said controlling means includes a capacitor, means forapplying a voltage across said capacitor which is dependent upon theamplitude of a sample, and means for controlling the drawing of a lineby said pen means of a length which is proportional to the magnitude ofthe voltage across said capacitor.
 20. A strip chart recorder inaccordance with claim 19 further including means for adjusting the baseline of said bar graph and the average value of the trace recorded onsaid paper relative to each other such that the separation andsuperposition of said trace and said bar graph can be varied.
 21. Astrip chart recorder in accordance with claim 20 wherein said paperincludes mutually perpendicular graticule markings, said pen means ismoved fast enough during the recording of a line such that saidgraticule markings are readily observable in the region of said bargraph, and the intensity of said trace is great enough such that thedrawing of said lines with substantially the same intensity wouldobscure said graticule markings.
 22. A strip chart recorder inaccordance with claim 21 wherein said first signal is derived from thecontinuous cardiographic monitoring of a patient, and the samples ofsaid second signal are proportional to a count of the number of eventsof a specified nature which occur between the taking of successivesamples.
 23. A strip chart recorder in accordance with claim 1 furtherincluding means for adjusting the base line of said bar graph and theaverage value of the trace recorded on said paper relative to each othersuch that the separation and superposition of said trace and said bargraph can be varied.
 24. A strip chart recorder in accordance with claim23 wherein said paper includes mutually perpendicular graticulemarkings, said pen means is moved fast enough during the recording of aline such that said graticule markings are readily observable in theregion of said bar graph, and the intensity of said trace is greatenough such that the drawing of said lines with substantially the sameintensity would obscure said graticule markings.
 25. A strip chartrecorder in accordance with claim 24 wherein said first signal isderived from the continuous cardiographic monitoring of a patient, andthe samples of said second signal are proportional to a count of thenumber of events of a specified nature which occur between the taking ofsuccessive samples.
 26. A strip chart recorder in accordance with claim1 wherein said paper includes mutually perpendicular graticule markings,said pen means is moved fast enough during the recording of a line suchthat said graticule markings are readily observable in the region ofsaid bar graph, and the intensity of said trace is great enough suchthat the drawing of said lines with substantially the same intensitywould obscure said graticule markings.
 27. A strip chart recorder inaccordance with claim 26 wherein said first signal is derived from thecontinuous cardiographic monitoring of a patient, and the samples ofsaid second signal are proportional to a count of the number of eventsof a specified nature which occur between the taking of successivesamples.
 28. A strip chart recorder in accordance with claim 1 whereinsaid first signal is derived from the continuous cardiographicmonitoring of a patient, and the samples of said second signal areproportional to a count of the number of events of a specified naturewhich occur between the taking of successive samples.